Field of the Invention
The invention lies in the communications field and relates, more specifically, to a method and a radio station for data transmission via a radio interface in a GSM mobile radio system.
U.S. Pat. No. 4,914,651 (European patent application EP 0 360 589 A) describes a mobile radio system in which additional channels having a bandwidth of 2 kHz are arranged between each two normal AMPS channels having a bandwidth of 30 kHz.
In radio communications systems, which are also referred to as wireless communications systems, data (for example voice, image information, or other data) are transmitted as signals with the aid of electromagnetic waves. The electromagnetic waves travel via a radio interface between a transmitting and a receiving radio station (base station and mobile station, respectively). The electromagnetic waves are in this case transmitted at carrier frequencies which are in the frequency band envisaged for the respective system. In the case of GSM (Global System for Mobile Communication), the carrier frequencies are currently in the range of 900, 1800 or 1900 MHz. Use in other frequency bands is equally possible.
During their propagation in a propagation medium, the signals are subject to interference from noise. As a result of diffraction and reflections, signal components traverse different propagation paths and are superposed in the receiver, where they lead to cancellation effects. Furthermore, if there is more than one signal source, these signals are superposed. Frequency division multiple access (FDMA), time division multiple access (TDMA), or a method known as code division multiple access (CDMA) is used to distinguish between the signal sources and thus to separate the signals at the receiver.
The GSM mobile radio system which is in existence at the present time is a radio communications system using a TDMA component for subscriber separation (Time Division Multiple Access). Useful information of the subscriber connections is transmitted in time slots in accordance with a frame structure. The transmission takes place in blocks. For modulation, use is made of a GMSK modulation method (Gaussian minimum shift keying) with a modulation bandwidth of BT=0.3. The modulation bandwidth BT is a bandwidth/time product which, according to GSM 05.04 Version 4.0.3, pp. 7-21, designates the modulation method. The modulation bandwidth BT is a value referring to the bandwidth B available for the transmission of a bit of length T. Additional information may be found in J. Biala, xe2x80x9cMobilfunk und intelligente Netzexe2x80x9d [Mobile radio and intelligent networks], Vieweg Verlag, 1995, pp. 104-06.
On account of the spectral properties of this modulation, two adjacent channels of the radio interface cannot be used in the same radio cell. A GSM channel has a bandwidth of 200 kHz. As illustrated in FIG. 3, at least one channelxe2x80x94the one drawn using a broken linexe2x80x94cannot be used on account of adjacent channel interference.
The rapidly increasing number of subscribers in GSM mobile radio networks means that the network operators are reaching the limits of the capacity of their networks, since only a limited frequency spectrum is available to them. Any increase in the capacity of a GSM network would therefore be advantageous.
The object of the invention is to provide a data transfer method and radio station which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this kind, and which increases the capacity of a GSM mobile radio network.
With the above and other objects in view there is provided, in accordance with the invention, a method of data transmission via a radio interface in a mobile radio system, which comprises:
modulating transmission signals at a transmission end for data to be transmitted via a given connection, with a first modulation bandwidth, and using a channel with a first bandwidth for the connection;
determining at least one quality parameter of a transmission quality of a data transmission of the connection; and
selectively altering the modulation bandwidth for the connection in dependence on the quality parameter, and subsequently using a channel with a second bandwidth for the connection.
In other words, at least one quality parameter of the transmission quality of the data transmission of a connection is determined and the modulation bandwidth for the connection is altered as a function of the quality parameter. This allows adaptation of the bandwidth, and thus of the adjacent channel interference, in a manner which is appropriate to the requirements and to the situation. The bandwidth can be reduced progressively, with the result that in the same radio cell or in adjacent radio cells, on account of the reduction in adjacent channel interference, the capacity is increased by the utilization of additional channels.
The quality parameters which may be included in the process are, in accordance with various features of the invention and by way of example:
a number of channel coefficients of a channel pulse response for the connection which contain a predeterminable proportion of the total energy of the channel pulse response;
a signal level;
a number of detection errors (e.g. RXQUAL as a GSM-standardized representation of the bit error rate).
These quantities can be utilized particularly advantageously for quality statements since they are regularly determined and evaluated in the course of reception-end evaluation of the transmitted signals. It is equally possible to use a combination of these quantities for decision-making purposes.
The detection errors are advantageously determined by evaluation of a training sequence. Consequently, the quality parameter is particularly reliable since the training sequence is known at the reception end and a correct comparison value is available.
If dynamic channel allocation is used, the channel with a reduced bandwidth can be allocated as a function of the transmission quality. If the quality deteriorates, then the modulation bandwidth is increased once more. If the process is applied to a plurality of mobile stations, then this increases the total capacity within the radio cell since, among the many mobile stations, more than one will always have transmission conditions suitable for bandwidth reduction.
In accordance with an additional feature of the invention, the modulation is a GMSK modulation at a modulation bandwidth of BT less than 0.3 or a GMSK modulation at a modulation bandwidth of BT=0.3.
In this development of the novel method for data transmission via a radio interface in a GSM mobile radio system the data of the transmission signals are modulated with CPM modulation at the transmission end. A bandwidth is produced which is narrower than in the case of GMSK modulation with a modulation bandwidth of BT=0.3. Consequently, at least one channel with a reduced bandwidth is provided by the radio interface. The reduction of the modulation bandwidth goes beyond the permitted tolerances according to GSM 05.04 Version 4.0.3, pp. 7-21. Detection of the data is carried out at the reception end. This detection can be effected without distinguishing between channels with different bandwidths. This last also allows downward compatibility with existing mobile stations.
As a result, account is taken of the continuous improvement in detectors at the reception end and of the reduction in the size of radio cells, which guarantee a sufficient data transmission quality even given a smaller modulation bandwidth. If the modulation bandwidth is reduced, it is possible to use additional channels within a radio cell. The capacity of the GSM mobile radio network is significantly increased. It is advantageous to use GSMK modulation with a modulation bandwidth of BT less than 0.3, but other CPM (continuous phase modulation) transmission methods according to C. E. Sundberg, xe2x80x9cContinuous Phase Modulationxe2x80x9d, IEEE Communication Magazine, Vol. 24, No.4, pp. 35-38, April 1986, can also be used in the GSM mobile radio system.
In accordance with again a further feature of the invention, the radio interface provides channels with at least two different bandwidths for data transmission. This means that existing circuitry can continue to be used but in addition transmission devices for channels with a reduced bandwidth are introduced into the radio station or existing transmission devices are matched to the smaller modulation bandwidth. As a result, two channel types with a different bandwidth exist alongside one another.
As a result of the channels with a reduced bandwidth, adjacent frequency bands in a radio cell are advantageously used for data transmission. The utilization of adjacent radio channels virtually doubles the capacity of the radio interface. Experiments have shown that a modulation bandwidth of about BT=0.15 permits a good transmission quality and utilization of adjacent frequency bands.
The method according to the invention can be applied particularly advantageously to microcells or interiors. These have a reduced cell radius so that, as a rule, the transmission conditions are good enough to use the reduced modulation bandwidth. Consequently, the transmission capacity of the radio interface can be increased by a number of at least two channels per 400 kHz contiguous bandwidth, i.e., given a GSM bandwidth of 200 kHz, a minimum spacing of the center frequencies of 600 kHz is dispensed with.
Furthermore, it is advantageous to allocate at least two channels for a connection, at least one channel having a reduced bandwidth. The data rate can thus be increased for this connection, which is of interest particularly for data services (i.e. no voice information).
With the above and other objects in view there is also provided, in accordance with the invention, a radio station for data transmission via a radio interface in a mobile radio system, comprising:
a modulation device for modulating transmission signals containing data to be transmitted, selectively at two different modulation bandwidths corresponding to transmission channels having different bandwidths;
a control device connected to the modulation device for changing a previously chosen bandwidth for a transmission of the data of a connection in dependence on a transmission quality of the data transmission of the connection; and
a high-frequency device connected to the modulation device for transmitting modulated transmission signals of a connection with the two different modulation bandwidths.
As noted above, the transmission quality of the data transmission is defined with one or more quality parameters.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and radio station for data transmission, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.